The present invention especially relates to cast part production using sand casting moulds which are produced by means of a negative-pressure moulding method. Casting methods for producing cast parts from metal, e.g., from alloys of iron, aluminium or magnesium are generally known. Typical casting methods which require sand casting moulds are gravity casting or low-pressure casting.
In the case of low-pressure casting, a moulding box with a sand casting mould which is under a vacuum or negative pressure is positioned over a pressurised casting device. The moulding box with the sand casting mould which is under a vacuum/negative pressure is then docked via the sprue opening of the moulding box on the furnace outlet of the pressurised casting device and connected to this in a non-positive locking manner. As a result of a controlled pressure build-up in the furnace interior, molten metal rises via a furnace riser pipe into the furnace outlet of the pressurised casting device and flows via the sprue opening of the moulding box into the sprue of the sand casting mould. The sprue of the sand casting mould leads into a gate region which distributes the flow of molten metal via a runner system and optimally fills the mould cavities of the sand casting mould. So that while the molten metal is flowing no turbulences develop or material-specific critical velocities are exceeded, which mechanically or chemically (oxidation processes) negatively influence the eventual component, the flow velocity of the molten metal is controlled via the pressure in the pressurised casting device. After casting has been carried out, the sand casting mould is allowed to cool until the cast part which is produced thereby has sufficiently solidified to the extent that this can be shaken out of the moulding box. The sand casting mould is for example guided over a vibrating table so that the sand separates from the cast part.
In the case of gravity casting, a sand casting mould is filled from the top with molten metal. The metal flows owing to gravity—mostly turbulently—into the sprue of the sand casting mould and is also distributed there in the gate region via distribution runners into the cavities of the casting mould. By inserting corresponding filters in the sprue of the sand casting mould a laminar flow can also be created during the gravity casting. The filters have the additional advantage that impurities or oxidation products can be filtered out of the molten metal as well.
The described low-pressure casting method or the gravity casting is used especially for the casting of light metals, e.g., aluminium alloys.
For casting on an industrial scale, it is important to operate the casting plant as efficiently as possible. Therefore, it is important to achieve short cycle times for producing the individual cast parts. An important factor for the cycle time is the cooling rate of the produced component. The quicker the cast part solidifies, or is sufficiently hard, in order to shake it out of the sand casting mould or the moulding box, the more efficiently can the casting plant be operated.
A faster quenching or solidifying of the molten metal, however, also brings with it improved mechanical properties for the produced cast part. In other words, as a result of a fast and targeted cooling of the melt metal structures with better mechanical properties are created (e.g., solidification of the cast part).
In order to accelerate the cooling of the sand casting mould, solutions from the prior art are already known.
Document U.S. Pat. No. 7,121,318 proposes that after the filling of a sand mould (a sandstone conglomerate with a binding agent) with molten metal, the sand mould comes into contact with a solvent, e.g., water. As a result, the molten metal is cooled more quickly in the boundary regions and begins to solidify there. The boundary region of the cast part with solidified surface also comes directly into contact with solvent in the process and is consequently further quenched. Due to the solvent, the sand casting mould is also dissolved in the cooled region. Document U.S. Pat. No. 7,121,318 proposes that the sand casting mould is immersed into a bath which is filled with the solvent.
In a further document, U.S. Pat. No. 7,216,691, it is proposed to spray a sand casting mould, filled with molten metal, with water or to immerse it into a water bath for the purpose of faster cooling of the casting mould or faster solidifying of the cast part which is contained therein. The aim in this case is also the dissolution of the sand casting mould. As a result of the targeted impingement of individual regions of the sand casting mould with the cooling agent, a zonally directed quenching is also achieved and therefore also zonally better mechanical properties in the produced cast part.
Document DE-11-2006-000627-T5 describes a further method and a sand casting mould with improved heat dissipation, preferably for producing cast parts from aluminium alloys. The sand mould and the cores consist of silica sand which are mixed with a water-soluble binding agent so that the silica sand remains in the desired form. For the purpose of targeted solidification of specific places on the cast part which is to be produced, water-soluble cores are inserted at corresponding places of the sand casting mould. If after the casting process has been carried out the sand casting mould is sprayed with a water jet at the places with inserted, water-soluble cores, then the water-soluble binding agent dissolves and the cores are washed away. As a result, not only a solidified boundary region forms on the cast part in a relatively short space of time at this place but the cast part is also quenched even faster as soon as the core is washed away and the water jet comes directly into contact with the solidified cast part surface. Consequently, a local solidification of the cast part can also take place.
Document U.S. Pat. No. 4,222,429 describes a cooling method for a vacuum sand casting mould. In this case, the sand casting mould, which is under negative pressure, is filled with molten metal. By means of the negative pressure in the sand casting mould which continues to exist as a result of the suction, gases which are possibly additionally created (evaporating styrene resin) during the casting process are discharged from the sand casting mould. For cooling the sand casting mould, a gas is then purged through the (porous) casting mould following the solidification of the cast part and then sucked out of it again, as a result of which an additional cooling or quenching of the casting mould and of the cast part is effected. Used as the cooling gas is for example air which is pumped by means of a compressor into the sand casting mould. Steam can also be used for purging instead of air. This purging of the sand casting mould creates a heat dissipation. In addition to the purging of the sand casting mould with a cooling gas or air, the sand casting mould can still be sprayed with water from the outside.